With the increase in density of optical disks in recent years, its information recording layer has higher linear recording density, and its tracks are provided at a smaller pitch. Such density increase in an optical disk also requires reduction in beam diameter of a light beam focused on the information recording layer of the optical disk. This can be achieved by an increase in numerical aperture of the light beam emitted from an objective lens serving as a light-harvesting optical system of the optical pickup device, or a decrease in wavelength of the light beam.
When the light beam passes through a cover glass of an optical disk, a spherical aberration is generated. The magnitude of the spherical aberration is generally proportional to a biquadrate of the numerical aperture, and therefore an error of the spherical aberration gives a certain influence to information recording when the objective lens has a high numerical aperture. For this reason, it is necessary to correct the spherical aberration when the objective lens has a high numerical aperture. The following describes some prior arts regarding detection of spherical aberration.
For example, Japanese Unexamined Patent Publication Tokukai 2002-55024 (published on Feb. 20, 2002) and Japanese Unexamined Patent Publication Tokukai 2000-171346 (published on Jun. 23, 2000) disclose a method of detecting a spherical aberration in a light-harvesting optical system using hologram diffraction ray. Also, Japanese Unexamined Patent Publication Tokukai 2002-157771 (published on May 31, 2002) discloses a method of detecting spherical aberration by appropriately dividing a light beam by a hologram. In this method, the difference between the positions where the spot diameters of the respective light beams are minimized is increased, which further increases the degrees of focal displacement of the respective light beams. In this manner, the spherical aberration can be detected at high sensitivity. Further, Japanese Unexamined Patent Publication Tokukai 2006-65935 (published on Mar. 9, 2006) discloses detection of a spherical aberration using a hologram with an optical pickup device having an optical integrated unit which is arranged such that the diameter of a light beam on a diffraction grating is increased to provide a longer light path between a diffraction element and a photoreceiver.
FIG. 11 is a drawing showing a layout of optical system components in a conventional optical pickup device 101. This optical pickup device 101 includes a semiconductor laser 1, a beam splitter 2, a polarization hologram 3, a transmission grating 4, a collimator lens 5, an objective lens 6 and a photoreceiver 8. FIG. 10 shows a pattern of the polarization hologram 3. FIG. 12 shows a structure of the photoreceiver 8. The light emitted from the semiconductor laser 1 (light source) passes through the beam splitter 2, and is incident on the polarization hologram 3. The incident light transmits through the polarization hologram 3, and is divided into three beams by a transmission grating 4 in a tangential direction, before being condensed onto the surface of the optical disk 7 by the collimator lens 5 and the objective lens 6. Passing through the collimator lens 5 and the objective lens 6, the reflection light from the optical disk 7 is again incident on the polarization hologram 3.
The polarizing directions of the light beams incident on the polarization hologram 3, i.e., the light from the light source and the reflection light of the optical disk 7 has a 90° difference which is given by a wavelength plate (not shown). As a result, the light beam from the semiconductor laser 1 passes through the polarization hologram 3, and the reflection light from the optical disk 7 is diffracted by the characteristic of the polarization hologram 3, and the resulting light beams are respectively focused onto the five separate light-receiving sections PD1 to PD5 shown in FIG. 12 in the regions 3a to 3c of the polarization hologram 3 shown in FIG. 10. FIG. 12 shows the photoreceiver 8 and the light beams condensed thereon.
As shown in FIG. 10, the polarization hologram 3 has a circular shape, and includes regions 3a to 3c. Among them, the region 3c is one of the semicircles divided in a radial direction by a center line. The regions 3a and 3b are included in the other one of the two semicircles of the polarization hologram 3. The region 3b is a semicircle smaller than the region 3c, and the region 3a surrounds the circular arc portion of the regions 3b. The region 3a is an area surrounded by a straight line in the radial direction orthogonal to the optical axis of the light beam, the first semicircle (the other one of the semicircles), and a second semicircle (the circular arc portion) which is concentric to a first semicircle and smaller in radius than the first semicircle.
FIG. 13 shows the diffraction by the polarization hologram 3. As shown in FIG. 13, the light-receiving sections PD1 to PD3 in the photoreceiver 8 are aligned at a predetermined interval along the tangential direction with the light-receiving section PD1 in the center. Meanwhile, the light-receiving sections PD1, PD4 and PD5 are aligned at a predetermined interval along the radial direction with the light-receiving section PD1 in the center.
The main beam MB in the center of the three divided light beams is guided to the light-receiving section PD1 as 0-th order diffraction ray which has passed through the polarization hologram 3, and is condensed as a light spot SP1 as shown in FIG. 12. Further, the sub-beams SB1 and SB2 on the both sides of the main beam MB among the three divided light beams are guided to the light-receiving sections PD2 and PD3, and are condensed as light spots SP2 and SP3. The main beam MB is guided to the light-receiving section PD4 as −1st order diffraction ray which has been generated in the region 3c of the polarization hologram 3, and is condensed as a light spot SP4. The main beam MB is also guided to the light-receiving section PD5 as a +1st order diffraction ray which has been generated in the region 3a of the polarization hologram 3, and is condensed on as a light spot SP5.
As shown in FIG. 12, the light-receiving section PD1 has four divided light-receiving regions A to D, and detects the 0-th order diffraction ray of the main beam having been passed through the polarization hologram 3. The light-receiving section PD2 has two divided light-receiving regions E and F, and detects one of the sub-beams. The light-receiving section PD3 has two divided light-receiving regions G and H, and detects the other of the sub-beams. The light-receiving sections PD2 and PD3 are used for generation of tracking servo signals. The light-receiving section PD4 has two divided light-receiving regions I and J, and detects the −1st order diffraction ray. The light-receiving section PD4 is used for detection of FES signals. The light-receiving section PD5 has two divided light-receiving regions K and L, and detects the +1st order diffraction ray. The light-receiving section PD5 is used for detection of spherical aberration signals (SA signals).
FIG. 12 shows only the +1st order diffraction ray as the light diffracted by the region 3a, and shows only the −1st order diffraction ray as the light diffracted by the region 3c. The light diffracted by the region 3b of the polarization hologram 3 is not discussed here, and therefore not shown in the figure.
When a spherical aberration occurs due to a thickness error of the optical disk 7, as shown in FIGS. 14(a) and 14(b), the light beams focused onto the light-receiving regions K and L of the light-receiving section PD5 form light spots SP5a and SP5b. In each of them, the light-receiving area in the light-receiving region K or L is larger than the light-receiving area of the other light receiving region. Further, if a spherical aberration does not occur, the light beam focused onto the light-receiving regions K and L form a dot light spot SP5c on the interface between the light-receiving regions K and L as shown in FIG. 14(c). It however should be noted that the influence of defocus is not taken into account. Here, expressing the electric signals generated in the light-receiving regions K and L respectively as Sk and Sl, the difference Sk−Sl of these electric signals is calculated. According to the calculation, the signal of Sk−Sl becomes 0 when a spherical aberration does not occur, but the signal of Sk−Sl becomes a positive value or a negative value when a spherical aberration occurs, and therefore a spherical aberration can be detected as a signal.
However, in the conventional optical pickup device 101, the objective lens 6 is shifted in the radial direction (vertical to the track) so as to allow the light spot to follow a concentric or helical track formed on the optical disk 7. With this shifting of the objective lens 6, the spot SPH on the polarization hologram 3 derived from the reflection light of the optical disk 7 is also shifted in the radial direction from the state of FIG. 15(b) into the state of FIG. 15(a) or 15(c). This affects the shapes of the light spot SP5d or the light spot SP5e diffracted onto the light-receiving regions K and L, as shown in FIG. 16(a) or 16(c). This shifting of the objective lens 6 causes a change of shape of the optical light spot, thereby changing the electric signals in the light-receiving regions K and L. Therefore, such shifting of the objective lens 6 causes generation of positive or negative Sk−Sl signals, in contrast to the light spot SP5f of FIG. 16(b) where the objective lens 6 is not shifted. This is called an offset signal, hereinafter.